An example of conventional thermal flow rate sensors includes, as described in Patent Document 1: a main flow path through which a fluid flows; and a sensor flow path that branches off from the main flow path so as to cause the fluid to flow in a split flow and is provided with a flow rate detecting mechanism configured to detect a flow rate of the fluid.
An example of the flow rate detecting mechanism is configured so that, as shown in FIG. 10, a sensor flow pipe forming the sensor flow path is provided with an upstream-side coil and a downstream-side coil that are independent of each other and that are each configured by using a heat-generating resistor of which the resistance value changes in accordance with temperature, the flow rate detecting mechanism being further provided with: an upstream-side constant temperature controlling circuit that has a bridge circuit including the upstream-side coil; and a downstream-side constant temperature controlling circuit that has a bridge circuit including the downstream-side coil. Further, the constant temperature controlling circuits control the temperatures of the upstream-side coil and the downstream-side coil so as to be equal to each other and to be constant at all times. The flow rate detecting mechanism is configured to calculate a flow rate Q based on a formula such as, for example, (Vu−Vd)/(Vu+Vd) or (Vu−Vd), where Vu denotes an output voltage of the upstream-side constant temperature controlling circuit, whereas Vd denotes an output voltage of the downstream-side constant temperature controlling circuit.
In other words, the thermal flow rate sensor is configured in such a manner that, when a fluid flows through the sensor flow path, because heat is taken from the upstream-side coil by the fluid, the electric power required to keep the temperature of the upstream-side coil constant increases, and the output voltage Vu of the upstream-side constant temperature controlling circuit therefore becomes higher. In contrast, because heat is given to the downstream-side coil by the fluid, the electric power required to keep the temperature of the downstream-side coil constant decreases, and the output voltage Vd of the downstream-side constant temperature controlling circuit therefore becomes lower.
Incidentally, when a gas having high thermal conductivity (e.g., He, H2, or the like) is used, because the amount of heat taken from the upstream-side coil provided on the sensor flow path is efficiently transferred to the downstream-side coil provided on the sensor flow path, the linearity of flow rate output characteristics of the thermal flow rate sensor with respect to the flow rate of the fluid flowing through the sensor flow path is relatively good.
However, when a gas having low thermal conductivity (e.g., SF6, CF4, or the like) is used, because the efficiency of the heat transfer is low in transferring the amount of heat taken from the upstream-side coil provided on the sensor flow path to the downstream-side coil provided on the sensor flow path, the linearity of the flow rate output characteristics of the thermal flow rate sensor with respect to the flow rate of the fluid flowing through the sensor flow path is not satisfactory. In particular, a problem is that, as the flow rate of the fluid flowing through the sensor flow path increases, the output voltage Vd of the downstream-side constant temperature controlling circuit tends to become saturated so that the linearity deteriorates. The degree by which the linearity deteriorates is different for each thermal flow rate sensor. Thus, a problem arises where, when the standard curve of each thermal flow rate sensor is corrected by using the standard curve of a thermal flow rate sensor used as a reference, it is difficult to accurately perform, for example, a span correction on the standard curves by performing a one-point correction. In other words, when the linearity of the standard curve of each thermal flow rate sensor is not maintained, even if a one-point correction is made, the level of precision of the corrections in the parts other than the one point is not satisfactory. As a result, a problem arises where the level of precision of the process to measure an actual gas flow rate deteriorates.
The abovementioned problems occur not only with thermal flow rate sensors employing a constant temperature controlling method, but also with thermal flow rate sensors that employ a constant current controlling method and that include, as shown in FIG. 11, a bridge circuit structured by connecting an upstream-side coil and a downstream-side coil in series and a constant current circuit that causes a constant current to flow through the bridge circuit.